The way it was taught to me was that each technique corresponds to a response to an attach. All 17 cover all the basics.

For example:
1 Shomen ate: Strike, same posture as attacker, inside.
2 Aigamae ate: Strike, same posture as attacker, outside.
3 Gyaku gamae ate: Strike, opposite posure, outside.
4 Gedan ate: Strike, opposite posure, inside.
5 Ushiro ate: read strike.
gives you all the different strikes from different positions that one can do. They are __BASIC__ strikes and not an exhaustive list. Same things can apply to all the other techniques in the 17 kata. After that, randori teaches you to apply those into a more realistic situation. Each of the 17 techniques can be applied in dozens of different ways.